Semiconductor manufacturing, such as the fabrication of integrated circuits, typically entails the use of photolithography. A semiconductor substrate on which circuits are being formed, usually a silicon wafer, is coated with a material, such as a photoresist, that changes solubility when exposed to radiation. A lithography tool, such as a mask or reticle, positioned between the radiation source and the semiconductor substrate casts a shadow to control which areas of the substrate are exposed to the radiation. After the exposure, the photoresist is removed from either the exposed or the unexposed areas, leaving a patterned layer of photoresist on the wafer that protects parts of the wafer during a subsequent etching or diffusion process.
The photolithography process allows multiple integrated circuit devices or electromechanical devices, often referred to as “chips,” to be formed on each wafer. The wafer is then cut up into individual dies, each including a single integrated circuit device or electromechanical device. Ultimately, these dies are subjected to additional operations and packaged into individual integrated circuit chips or electromechanical devices.
During the manufacturing process, variations in exposure and focus require that the patterns developed by lithographic processes be continually monitored or measured to determine if the dimensions of the patterns are within acceptable ranges. The importance of such monitoring, often referred to as process control, increases considerably as pattern sizes become smaller, especially as minimum feature sizes approach the limits of resolution available by the lithographic process. In order to achieve ever-higher device density, smaller and smaller feature sizes are required. This may include the width and spacing of interconnecting lines, spacing and diameter of contact holes, and the surface geometry such as corners and edges of various features. Features on the wafer are three-dimensional structures and a complete characterization must describe not just a surface dimension, such as the top width of a line or trench, but a complete three-dimensional profile of the feature. Process engineers must be able to accurately measure the critical dimensions (CD) of such surface features to fine tune the fabrication process and assure a desired device geometry is obtained.
As a result, careful monitoring of surface roughness for features is becoming increasingly important. As design rules shrink, the margin for error in processing becomes smaller. Even small deviations from design dimensions may adversely affect the performance of a finished semiconductor device. Characteristics such as profile roughness, such as roughness along the bottom of trenches or on the sidewalls of lines, which could be ignored for larger features, now consumes a large amount of the tolerance budget.
In this application, the phrase profile roughness will include both roughness on one edge of a feature, referred to as line edge roughness (LER), and roughness of the entire feature, referred to as line width roughness (LWR). It should be recognized that the terms “line edge roughness” and “edge roughness” are often used to refer to roughness characteristics of structures other than just lines. For example, the roughness characteristic of a two-dimensional structure, such as a via or hole, is also often referred to as a line edge roughness or edge roughness. In this application, the following description, the terms “line edge roughness” and “edge roughness” are also used in this broad sense.
A number of different methods of measuring profile roughness are known in the prior art. Various optical methods, such as optical profilometry or scatterometry, can be used to rapidly determine surface roughness. However, the resolution of optical methods is limited, typically greater than 0.5 micron, and such methods do not directly measure surface topography.
Mechanical profilers, such as scanning probe microscopes, can be used to generate very detailed three-dimensional measurements of surface roughness. However, mechanical profilers are typically very slow. Resolution is limited by the size of the probe or stylus, and very small probes are difficult to manufacture and are very fragile. Also, measurement of features with high-aspect ratios or undercut surfaces is very difficult using any type of stylus profilometer.
Some types of profile roughness, such as LER and LWR, can be monitored using electron beam techniques. The scanning electron microscope (SEM) allows for the production of an image of greater magnification and higher resolution than can be achieved by the best optical microscopes. An SEM produces a finely focused beam of electrons, which is scanned across the surface of a work piece, typically in a raster pattern. The electrons that make up the electron beam are called primary electrons. When the electron beam is directed at the work piece surface, the primary electrons collide with electrons in orbit around the nuclei of the atoms present in the work piece causing the emission of secondary electrons. Some of the primary electrons will also be reflected from the work piece surface. These higher energy electrons (>50 eV) are called backscattered electrons. Both types of electrons can be detected by inserting an appropriate detector near the specimen. The detector produces a variable voltage output; the more secondary or backscattered electrons it detects, the greater will be the voltage generated.
Typically, to measure the width of a structure, the SEM is used in conjunction with automatic metrology software. As the electron beam is scanned across the exposed cross-section, whether secondary or backscattered detection is employed, there will typically be a change in electron intensity at the edges of the structure. This change can be due to a change to topography or to a transition between two different materials. An algorithm is used to assign an edge position based upon the contrast at the edges of the structure and to determine the distance between those edges.
The SEM alone, however, can only view a feature from the top down. While overall roughness can be measured, it is very difficult to determine whether the roughness is at the bottom or top of the feature. Further, the SEM, like the optical and mechanical methods discussed above, can only be used to measure surface features. The profile roughness of buried features or features surrounded by other materials cannot be measured using these methods.
It is possible to get more accurate information of a feature profile and to measure buried features by using a charged particle beam system, such as a focused ion beam system (FIB), in conjunction with a scanning electron microscope (SEM). FIB systems are widely used in microscopic-scale manufacturing operations because of their ability to image, etch, mill, deposit, and analyze very small structures with great precision. FIB systems produce a narrow, focused beam of charged particles (hereinafter referred to as ions) that is typically scanned across the surface of a work piece in a raster fashion, similar to a cathode ray tube. In most commercial FIB systems, the ions used are positively charged gallium ions (Ga+) extracted from liquid metal ion sources. The extracted ions are accelerated collimated, and focused onto a work piece by a series of apertures and electrostatic lenses. The ion beam can be used to remove material from the work piece surface or to deposit material onto the surface. When used to remove material, often referred to as milling, the heavy gallium ions in the focused ion beam physically eject atoms or molecules from the surface by sputtering, that is, by a transfer of momentum from the incoming ions to the atoms at the surface.
The FIB system can be used to expose the cross-section of a feature, so that the profile of the feature can be accurately measured. Once the cross-section is exposed, a scanning electron microscope can be used to measure the profile of the feature. This measurement, however, is still only a two dimensional measurement at one particular point. Three-dimensional measurement is required to understand the presence of dimensional variations such as standing waves or changes in slope and to adequately control all of the factors that contribute to increase in roughness, such as pattern transfer, deposition, or planarization.
Thus, there is still a need for an improved method of measuring the three-dimensional surface roughness of a semiconductor feature.